The present invention relates to a method for preparing an electrolytic solution, an electrolytic solution and an electric double-layer capacitor. It relates particularly to a method for preparing an electrolytic solution capable of preventing overcharge of an electric double-layer capacitor, an electrolytic solution and an electric double-layer capacitor.
Electric double-layer capacitors, which have high capacitance of farad class, excellent characteristics of charge/discharge cycle and the capability of undergoing rapid charge, have been used for a backup power supply of electronic component, an onboard battery for vehicle (an energy buffer) and the like.
An electric double-layer capacitor will be briefly described referring to FIG. 1.
FIG. 1 is a sectional view showing the main structure of an electric double-layer capacitor.
As shown in FIG. 1, an electric double-layer capacitor 101 includes a casing 102 housing a pair of carbon electrodes (polarizing electrodes) 104 which interposes a separator 103, and a pair of collectors (elements) 105. And the casing 102 is filled with an ion conductive electrolytic solution. The electric double-layer capacitor 101 employs electric charges (shown by + and xe2x88x92 in FIG. 1) as dielectrics in a normal capacitor, which are generated at an interface between the solid carbon electrodes 104 and the liquid electrolytic solution and spaced at a distance of molecule.
Electrolytic solutions used for electric double-layer capacitors are roughly categorized into an aqueous electrolytic solution made of dilute sulfuric acid added with an electrolyte and an organic electrolytic solution made of an organic solvent added with an electrolyte. An appropriate type of electrolytic solution is selectably applied to an electric double-layer capacitor taking into account the usage thereof. An electric double-layer capacitor using an aqueous electrolytic solution is advantageous in terms of lower internal resistance and higher power density, which also enables flexibility for setting of voltage requirements. On the other hand, an organic electrolytic solution, which allows a higher withstand voltage per cell, is advantageous in terms of energy density. It also allows selection of inexpensive and light metals such as an aluminum alloy for a casing.
An activated carbon or activated carbon fiber with a large specific surface area is generally used for an electrode of electric double-layer capacitor so that the capacitor can attain high electrostatic capacitance. Since the more the specific surface area of an activated carbon increases, the more the number of pores therein will increase, the amount of adsorption of electrolytic ions grows, thereby resulting in higher electrostatic capacitance. Generally speaking, an activated carbon having a specific surface area of some thousands square meters per gram (m2/g) is used for an electrode. It is reported that application of an activated carbon with a large specific surface area can provide an electric double-layer capacitor with high capacitance such as some hundreds to some thousands farads (F).
Volume and specific surface area per weight for an activated carbon are substantially linearly proportional each other. However, volume per electrode reaches maximum when the specific surface area of an activated carbon falls into the range of 2000 to 2500 m2/g, and decreases if the area exceeds the range, which is reported in the document (DENKI KAGAKU, 59, P.607). The reason for it is that the density of an electrode seemingly decreases due to an increase in volume of pores according as a specific surface area increases. Improvement of the specific surface area of an activated carbon is believed to relate closely to higher electrostatic capacitance. However, as the bulk density of an activated carbon decreases with the increase of pores in connection with improvement of specific surface area, electrostatic capacitance per volume of an electrode will accordingly fall.
In an effort for increasing the electrostatic capacitance per volume of an electrode, the development of an electrode using a graphitized carbon has been started.
Especially, an activated carbon (mesophase carbon fiber or mesophase microsphere), which is made of alkali activated mesophase pitch, has been recently used for a polarizing electrode of electric double-layer capacitor.
Mesophase carbon fiber is a kind of carbon fiber which is produced from pitch and the like as an ingredient (graphitized carbon). Pitch is optically isotropic, but when it is heated, pitch molecules start regular orientation, thereby a portion of optical anisotropy (optically anisotropic microsphere) is generated. Eventually, the portion is transformed into coke completely, which is optically anisotropic and shows a flow pattern. Such a portion of optical anisotropy is called mesophase. Mesophase carbon fiber is a type of carbon fiber produced from pitch, which is transformed into mesophase to some extent, by spinning such as melt-blow method.
Mesophase carbon fiber has optical anisotropy and relatively high degree of orientation. The basic orientation is immune to oxidization (infusible treatment) by air, and what is more it is remarkably improved by carbonization and high temperature treatments. Also mesophase carbon fiber has high graphitization. In this way mesophase carbon fiber can be applied to a polarizing electrode having high electrostatic capacitance per volume.
Japanese Patent Application Publication 05-258996 discloses an electrode employing mesophase carbon made of carbonaceous fiber, which is activated by an aqueous solution of alkaline metal hydroxide and crushed. The carbonaceous fiber is produced from pitch with melt spinning and subsequent heat treatment. Also Japanese Patent Application Publication 09-275042 discloses a polarizing electrode with high electrostatic capacitance using an activated carbon, which is produced from vinyl chloride resin with baking and subsequent alkali activation.
An electric double-layer capacitor arranged as described above is adaptive for rapid charge when the capacitor is operated under cyclic charge and discharge. Generally, the capacitor is operated while electrically connected to a charge and discharge control circuit or to an overcharge protection circuit in order to prevent overcharging. When the voltage of an electric double-layer capacitor exceeds a predetermined value, namely a predetermined charge control voltage, the circuit cuts off power supply to the capacitor.
However, when there is not provided a circuit of this type or the function of overcharge protection of the circuit does not work properly, the performance of charge and discharge of the electric double-layer capacitor may deteriorate, which is overcharged with a higher voltage than the predetermined value.
Furthermore as described before, when electrostatic capacitance per volume is increased, the volume of an electrode generally tends to expand. It accordingly requires a reduction in filling factor of electrode. If the filling factor is thus set smaller, which leads to a reduction in energy density, an electric double-layer capacitor cannot fully demonstrate its features. Typically, this holds true of an electric double-layer capacitor which employs a material with high electrostatic capacitance for an electrode, such as an activated carbon produced from mesophase pitch with alkali activation.
Instead of reducing the filling factor, it may be possible to increase the thickness of casing for an electric double-layer capacitor so that the casing can withstand the load generated by expansion of the electrode.
However, the increase in casing thickness results in an undesirable increase in the gross weight of the capacitor.
An object of the present invention is to provide a method for preparing an electrolytic solution and the electrolytic solution which can prevent overcharging of an electric double-layer capacitor.
The other object of the present invention is to provide an electric double-layer capacitor using the electrolytic solution which facilitates protection against an overcharge for the capacitor.
The inventors of the present invention have made strenuous efforts towards achieving an electrolytic solution which is able to prevent overcharging of an electric double-layer capacitor. As a result of study, it has been discovered that overcharging can be prevented by preparing an electrolytic solution so that the solution turns to a nonconductor at a predetermined voltage. The voltage is determined so as to be equal to or greater than the upper limit of operating range of voltage and less than or equal to maximum allowable voltage of the electrolytic solution.
An object of the present invention is to provide a method for preparing an electrolytic solution of an electric double-layer capacitor which undergoes cyclical charging and discharging within a predetermined operating range of voltage. The capacitor includes the electrolytic solution, electrodes and a casing for housing the electrolytic solution and electrodes. The method has the step of preparing an ion concentration of the electrolytic solution so that the solution turns to a nonconductor at a first predetermined voltage, which is so set as to be equal to or grater than an upper limit of the predetermined operating range of voltage and less than or equal to a maximum allowable voltage of the capacitor.
As the electrolytic solution is prepared as described above, when the electric double-layer capacitor is overcharged at a voltage higher than the operating range of voltage, no ions exist in the electrolytic solution at the first predetermined voltage, which is less than or equal to the maximum allowable voltage. The electrolytic solution thus turns to a nonconductor so that charging of the capacitor is stopped. Therefore, the capacitor will not be charged if the voltage of electrolytic solution exceeds the first predetermined voltage.
In the present invention, the term xe2x80x9ca predetermined operating range of voltagexe2x80x9d used in the appended claims refers to a voltage range, under which a capacitor is normally used. xe2x80x9cA maximum allowable voltagexe2x80x9d means the maximum voltage of a capacitor, which is defined taking into account the characteristics of a polarizing electrode, casing and the like. The maximum allowable voltage may be set flexibly taking into account practical reasons such as the operating range of voltage for a capacitor, the external voltage imposed on the capacitor during charging and protection against a liquid leak.
Also the expression in the present invention xe2x80x9cthe ion concentration at which an electrolytic solution turns to a nonconductorxe2x80x9d means that the ion concentration is substantially close to zero in the electrolytic solution at the first predetermined voltage. In other words, it refers to an ion concentration at which the electric resistance of electrolytic solution is substantially large (is close to infinite).
Another object of the present invention is to provide a method for preparing an electrolytic solution of an electric double-layer capacitor, wherein the capacitor is employed while electrically connected to a charge and discharge control circuit having a second predetermined voltage thereof, and the first predetermined voltage is so set as to be equal to or greater than the second predetermined voltage and less than or equal to the maximum allowable voltage.
When the electric double-layer capacitor according to the present invention is used while connected to such a control circuit as the charge and discharge control circuit, the protection against overcharging can be implemented by both the first predetermined voltage of electrolytic solution and the second predetermined voltage of control circuit. In case failure occurs in the circuit, the failsafe arrangement can securely prevent overcharging of the capacitor.
Still another object of the present invention is to provide a method for preparing an electrolytic solution for an electric double-layer capacitor, wherein the ion concentration is set based on a function of charge voltage vs. coefficient of expansion of an electrode or the other function of charge voltage vs. generated load thereof.
When the ion concentration of electrolytic solution is set based on a curve representing the relation of charge voltage vs. coefficient of expansion of electrode, the filling factor of electrode can be determined taking into account the coefficient of expansion of electrode, thereby resulting in a increase in energy density of the electrode. On the other hand, when the ion concentration is set based on a curve representing the relation of charge voltage vs. generated load, the casing can be optimized. This leads to a reduction in the gross weight of the electric double-layer capacitor as a result of a reduction in the thickness of casing when one particular material is under consideration. Also this achieves a more compact electric double-layer capacitor by thinning the thickness of casing.
The term in the present invention xe2x80x9ccoefficient of expansion of an electrodexe2x80x9d is defined as the rate of increase in thickness of electrode while charged under a constant load. The term xe2x80x9cgenerated loadxe2x80x9d refers to a load generated while the electrode is charged, kept its thickness constant.
Yet another object of the present invention is to provide a method for preparing an electrolytic solution of an electric double-layer capacitor, wherein the electrodes are made of activated carbon which is produced from a graphitized carbon with carbonization and subsequent alkali activation.
Activated carbon produced from mesophase pitch with alkali activation has high electrostatic capacitance. When the activated carbon is used for a polarizing electrode, it is possible to provide an electric double-layer capacitor of high electrostatic capacitance with compact size. The method for preparing electrolytic solution according to the present invention can be applied to an electric double-layer capacitor, which employs activated carbon that is produced from a graphitized carbon such as mesophase carbon with carbonization and subsequent, alkali activation. Since the maximum values of coefficient of expansion and generated load for the activated carbon can be controlled, it is possible to relax the requirements for strength of casing and the like.
A further object of the present invention is to provide an electrolytic solution for an electric double-layer capacitor, to which the method for preparing an electrolytic solution is applied.
The electrolytic solution for electric double-layer capacitor according to the present invention turns to a nonconductor at a predetermined voltage, which cuts off charging of the capacitor, thereby preventing the capacitor from being charged when the voltage of electrolytic solution exceeds the predetermined voltage.
A still further object of the present invention is to provide an electric double-layer capacitor which undergoes cyclical charging and discharging within a predetermined operating range of voltage. The capacitor includes an electrolytic solution, electrodes and a casing for housing the electrolytic solution and electrodes. The capacitor has a feature that the electrolytic solution is prepared according to the method described before.
When the electric double-layer capacitor is overcharged at a higher voltage than the upper limit of operating range of voltage, ions will cease to exist in the electrolytic solution at a predetermined voltage for the electrolytic solution, which is set less than or equal to a maximum allowable voltage. The electrolytic solution thus turns to a nonconductor, thereby stopping charging of the capacitor. Therefore, charging of the capacitor at a higher voltage than the predetermined voltage can be prevented.
A yet further object of the present invention is to provide an electric double-layer capacitor. The capacitor has a feature that electrodes are made of activated carbon which is produced from graphitized carbon with carbonization and subsequent alkali activation.
According to the present invention it is possible to control the maximum values such as coefficient of expansion and generated load of mesophase carbon. It results in a better balance between the strength of casing and the filling factor of polarizing electrodes, thereby providing an electric double-layer capacitor having larger capacitance. In other words, the present invention makes it feasible to provide an electric double-layer capacitor with advantages of better compactness, lighter weight and larger capacitance.